Known peristaltic pumps in medical devices for extracorporeal blood treatment generally consist of a rotor, a pump housing and a pump line arranged between these as a fluid line and they convey a defined volume of a medium, such as blood or dialysis fluid, from the low-pressure side, normally the arterial side, to the high-pressure side, normally the venous side. The peristaltic pump works by deforming and pinching the elastically deformable fluid line with the squeeze elements. These are pre-tensioned against the fluid line and are moved along the latter on rotation of the rotor, thereby causing elastic deformation of the fluid line in a cross-sectional direction. As a result of this, fluid is pressed out of the fluid line in the direction of conveyance. Replenishing fluid is drawn into the line with low pressure, in particular vacuum, which is created due to the elastic reshaping of the fluid line after deformation by the squeeze elements.
It is known that pre-tensioning of the squeeze elements against the fluid line can be effected with springs, for example. These are designed in such a way that the cross-section of the fluid line can be fully pinched, in other words the cross-section of the fluid line can be fully closed with the squeeze elements. If the pump functions properly, therefore, pressure differences are not transferred from the arterial side to the venous side.
When the conveyance volume is opened, i.e. at the transition of the fluid line cross-section from occlusive to non-occlusive, pulsation occurs on the high-pressure side. This effect is caused by the fact that on the low-pressure side a volume section of the elastic fluid line is pinched and the fluid volume enclosed inside it is conveyed in the direction of the high-pressure side with rotation of the rotor and the shifting of the squeeze point of the fluid line towards the high-pressure side. When the conveyance volume section is pinched, the volume enclosed inside it is under the pressure of the low-pressure side. It is conveyed under this pressure to the high-pressure side, where, by contrast, high pressure prevails. If the conveyance volume section is opened to the high-pressure side as part of the conveyance process, fluid flow occurs from the high-pressure side into the conveyance volume section due to the pressure difference between the high-pressure side and the conveyance volume section. This return flow lasts until pressure balance is established. The result is a momentary interruption of the pressure on the high-pressure side and a pulsation occurs on the high-pressure side.
In the case of a conveyance of blood as the fluid, it can occur during the pressure balance described above and as a result of the fluid line cross-section not being fully closed by the squeeze elements (insufficient sealing of the conveyance volume section) that blood is squeezed through the bottleneck that exists or is formed between the conveyance volume section and the high-pressure side. This normally results in the partial destruction of blood cells, generally referred to as haemolysis. In order to avoid haemolysis, the squeeze elements, the fluid line and the support surface of the peristaltic pump are harmonized in such a way that the cross-section of the fluid line is expanded as quickly as possible when opening the conveyance volume section to the high-pressure side, i.e. the transition between the states occluded and non-occluded is effected as swiftly as possible and also the cross-section is sealed as tightly as possible during conveyance.
It is a disadvantage of known methods for conveying fluid, in particular blood, that wear or failure can occur during the service life of such a peristaltic pump, leading to complete wearing out of the cross-section of the fluid line so that a swift transition from occluded to non-occluded cannot be ensured. Haemolysis can therefore not be reliably ruled out. It is further a disadvantage that a failure of a spring pre-tensioning a squeeze element against the fluid line in conventional methods can only be detected if it happens so completely that the required pressure levels can no longer be established. An undetected failure of the kind described here can potentially cause massive harm to a patient since increased haemolysis can occur.